Tandem-free operation (TFO) is a protocol that negotiates the removal of speech encoding and decoding functions within two entities engaged in a communication connection, if the encoder and the decoder types match. TFO can be applied, for example, in a core network supporting a mobile-to-mobile connection, resulting in a direct logical connection between the speech encoding and decoding functions of the mobiles. For more information on TFO protocols, the reader is invited to refer to the following documents, the contents of which are hereby incorporated by reference herein:    3rd generation partnership project, Technical specification group services and system aspects, Inband tandem free operation (TFO) of speech codecs, service description—Stage 3 (3GPP TS 28.062 V4.3.0 (2002-09));    3rd Generation Partnership Project; Technical Specification Group, GSM/EDGE Radio Access Network; (3GPP-TS 45.009 V4.2.0) Link adaptation    CDMA TFO standard TIA/EIA-895, “CDMA Tandem Free Operation”    CDMA TFO standard TIA/EIA/IS-893 “Selectable Mode Variable Service Option for Wideband Spread Spectrum Communication”
TFO can be used in the context of CDMA communication, in particular with a Selectable Mode Vocoder (SMV) codec. An SMV codec has several modes of operation. The normal modes are mode 0 (premium), mode 1 (standard), mode 2 (economy) and mode 3 (capacity-saving). In these modes all coding/decoding rates (full rate, ½ rate, ¼ rate and ⅛ rate) are used. However, there also exist mode 4 and mode 5, which use ½ rate, ¼ rate and ⅛ rate only, i.e., the full rate is not used. Proposed CDMA TFO negotiation protocols do not provide for codec capability exchange prior to entering TFO operation. Accordingly, a possibility exists that one entity that happens to be operating in mode 4 or 5 engages into TFO communication with another entity that is operating in mode 0, 1, 2 or 3. In this case, the entity operating in mode 4 or 5 is not capable of accepting full-rate data from the distant partner. This may lead to premature TFO establishment, unnecessary TFO termination and TFO failures.
Accordingly, a need exists in the industry to provide communication devices that support TFO operation and that are protected against TFO engagement when the two sides of the connection operate in incompatible modes.